1. Field of Invention
This invention relates to passive temperature compensation of rotatable gratings and other wavelength selecting devices used to tune lasers.
2. Prior Art
Laser radar (LIDAR) systems, utilizing tunable lasers, can be used to transmit different wavelengths of light into airborne suspensions (such as smog or poison gasses) which have differing reflectance""s or absorption to different wavelengths. The reflected light intensity is then measured for remote spectrographic analysis of suspension samples. It is advantageous to maximize the stability and repeatability of the output at each different wavelength. It is also advantageous to minimize intervals between transmitting wavelengths in order to reduce measurement interference by relative motion between the LIDAR unit, the intervening atmosphere and the suspension sample. Maximum accuracy is achieved by successively transmitting different wavelengths with constant power at the laser""s maximum cyclic rate.
Tunable lasers typically include an intra-cavity diffraction grating. The wavelength of such lasers is tuned by adjusting the angle of incidence of the laser cavity beam against the diffraction grating. Such intra-cavity tuning requires very high accuracy and stability. Tuned CO2 lasers, for instance, require a grating angular range of typically 0.2 radians and an accuracy of 10 or 20 xcexcradians. Output laser power is a sensitive function of the tuning angle near a particular wavelength. This accuracy can only be achieved through careful design of housings, optics, sensors, feedback loops, actuators and system configuration.
In FIG. 1 of the prior art, a representative directly driven grating implementation is shown. Intra-cavity beam 22 is tuned by rotary grating 24 responsively changing the Littrow reflection incidence angle relative to cavity 22, thereby selecting lasing wavelength. Actuator 20 in response to control by position control 28 rotates grating 24 to produce different wavelengths. Grating position is sensed by position sensor 32 and ambient temperature is sensed by sensor 31. Position control 28 includes position data for sequencing between desired positions and therefore wavelengths and provides output drive to actuator 20, responsive to said position data, actual position information from position sensor 32 and ambient temperature sensor 31.
Temperature induced variations, especially in the grating, position sensor, and sensor coupling pose a serious threat to the functionality of the tuner. These variations are the result of absorbed cavity as well as ambient energy. Military applications may require high cavity power and oppressively wide operating temperature ranges. Changes in temperature of the grating blank or base material, for instance, modify the apparent ruling of the grating and therefore its tuned frequency. The position sensor and its coupling have a direct effect on the tuned frequency of the cavity. Cavity detuning results in output power fluctuations.
Several techniques have been used to nullify the effects of these errors. Grating 24 can use a blank material of low coefficient of thermal expansion (CTE). Invar as a grating blank material, for instance, would have a CTE of typically 1.3 ppm/K. Special invar can reduce this somewhat further. Even this small CTE would generate an excessive error of 65 xcexcradians for a 40xc2x0 C. ambient rise and 10xc2x0 C. cavity induced temperature rise.
Another technique for ambient correction is to use the temperature sensor 31 to crudely modify the desired position address data in position control 28 in a calibrated way to compensate for the thermal induced errors of the grating and sensor. Thermal errors generated by laser cavity energy are not compensated unless complex rotating or additional non-contact thermal sensors are used.
These techniques are inordinately complex and do not function well even for the relatively small errors generated by low CTE gratings. Most serious of the deficiencies are the nonlinear and unpredictable temperature variations of most sensors and the thermal time constant mismatch between the elements. Additionally, invar is a poor material to work with. Invar has high density, low thermal predictability and low thermal conductivity. It is difficult to machine and has dissimilar metals incompatibility with many optical surface materials. Its density and thermal conductivity discourage use at high power or rapid tuning rates. The use of lighter, more compatible blank materials with unavoidably higher CTEs function even more poorly.
FIG. 2 of the prior art is based on an adaptive resonant positioner disclosed in U.S. Pat. No. 5,450,202. As disclosed, high accuracy and speed are combined as a result of the adjustment of adjacent pairs of drives on.a pattern delayed basis. Intra-cavity beam 22 is tuned by rotating agile mirror 42 which reflects beam 22 as beam 22a onto fixed grating 44 responsively determining the incidence angle between cavity 22a and grating 44 thereby selecting lasing wavelength. A second position of mirror 42 results in cavity beam 22b for another incidence angle and lasing wavelength. As shown, the cavity beams 22a and 22b intersect the grating at 26a and 26b for two of the tuned wavelengths in a tuning band. Mirror 42 is rotated by actuator 40 in response to control by agile position control 46. Mirror position is sensed by position sensor 48 and ambient temperature is sensed for calibrated correction by sensor 31. Agile position control 46 provides output drive to actuator 40 responsive to the internally defined desired position data, actual position information from position sensor 48 and ambient(temperature sensor 31.
This system operates better in a high tuning rate, thermally hostile environment for a number of reasons. Since the grating is fixed mounted, a massive and therefore thermally improved version including active thermal control 45 is possible. Alternately, even without thermal control 45, better thermal coupling reduces the thermal differences between ambient and grating temperature. More importantly, the mirror is no longer a thermally sensitive element and can be constructed of light material for high speed and compatibility with optical coatings. The position sensor is typically a linear Si sensor and is thus more predictable and stable than other types of sensors.
Although this approach enjoys speed and thermal advantages over the FIG. 1 prior art, it is still too complex, requires a larger grating for the same beam size, is susceptible to mismatched thermal time constants and falls short in performance for stringent military requirements. Wide operating temperature ranges still require the use of invar for grating blanks. Additionally, for high power tuners, the mirror damage threshold drops by one half since a reflection occurs twice as often for each cavity round trip.
Preferred embodiments of a passive thermal compensation for wavelength agile laser tuners according to the present invention include wavelength selection means, thermal shim means, sensor coupler means and tuner controller means. Thermal energy from lasing or environmental injection coupled into the wavelength selection means is tightly coupled to the thermal shim means. Temperature variations of the thermal shim means produce angular corrections to the operation of the wavelength selection means to produce corrected wavelength tuning at a typically narrow band of wavelengths and at a rate which closely tracks the error producing thermal expansion of the wavelength selector. Sensor coupler means receives thermal energy from environmental injection and modifies sensor tuning sensitivity to produce corrected tuning at a wider band of wavelengths.
LIDAR and other types of remote chemical sensors utilize the acute absorption of energy at specific wavelengths as compared with relatively low absorption at others. Unfortunately, laser return energy already includes unavoidable and confusing variations due to target and background motion, atmospheric scatter and sensor attitude changes. High repetition rates reduce some of the variations by tending to freeze the motion. It is important to maintain the transmitted energy as constant as possible in order that laser energy variations do not add to the return energy uncertainty. CO2 laser systems are especially sensitive since tuning angle variations of only a few xcexcradians can affect the lasing energy significantly. Techniques for rapidly and accurately tuning have been used successfully as described in U.S. Pat. No. 5,450,202 that utilizes a pseudo-resonant agile tuner controller.
Chemical sensors, especially in military applications, must operate in hostel environments. The effect of temperature on the tuning mechanisms is a particularly severe problem. The thermal energy comes from two primary sources; energy produced by the lasing process and ambient temperature. Temperature variations can easily exceed 50xc2x0 K on the basic tuning element. For some applications, even gratings based on invar blank technology produce unacceptably large tuning errors. For increased speed, gratings need to be based on non-invar, lighter and more materials compatible blanks. Such blanks have 2 to 10 times higher tuning thermal errors. Higher power applications also prefer better thermal conductivity, non-invar materials to avoid excessive thermal drops.
Objects and Advantages
It is a primary objective of the present invention to provide a simple temperature stable positioner suitable for wavelength tuning of high power agile lasers used in extreme thermal environments.
It is another objective to eliminate thermal sensors and complex correction algorithms.
It is another objective to compensate for both ambient and laser generated temperature variations.
It is another objective to provide a more accurate wavelength tuner.
It is another objective to provide more rapid wavelength tuning.
It is another objective to provide a simpler tuner.
It is another objective to enable the use of a wider range of materials for optical elements in a wavelength tuner.
It is another objective to reduce the number of intra-cavity reflections.
It is another objective to enhance compensation techniques by improving thermal time constant matching between thermal variations and compensation.
It is another objective to temperature compensate an OPO wavelength tuner.